Ferromagnetic metallic particles are used in high density magnetic recording media because they possess certain advantageous properties compared to conventional gamma iron oxide, cobalt modified iron oxide and chromium dioxide particles. These advantageous properties include higher coercivity, higher saturation magnetization and higher remanent magnetization which allow for recording of shorter wavelength signals with higher output signal levels. Coercivity of greater than 1000 Oe can be achieved for ferromagnetic metallic particles, whereas conventional cobalt modified iron oxides have a practical upper limit of coercivity of less than 1000 Oe due to instabilities, particularly changes in coercivity with changes in temperature and loss of signal amplitude with repeated playback. Ferromagnetic metallic particles have the disadvantage of being susceptible to oxidation with the passage of time which results in a decrease in the magnetization of the particles and potential signal loss in recorded media. This tendency toward oxidation also causes problems in handling since the particles may spontaneously ignite upon exposure to air if not properly stabilized.
Ferromagnetic metallic particles also have the disadvantage that, during the high temperature reduction process, the particles tend to sinter together and to change in size and shape. Such sintering and morphological changes result in degradation of magnetic properties.
It is therefore desirable to provide ferromagnetic metallic particles having superior magnetic properties and superior resistance to changes in magnetic properties on keeping.